As a method for recording digital data on optical disk media, there has commonly been employed a method of uniformizing the recording density on a recording medium by making the linear velocity constant, as seen in a compact disk (hereinafter referred to as a CD), a DVD (Digital Versatile Disk), and a DVD-RAM (Digital Versatile Disk-Random Access Memory). When reproducing a digital binary signal from a playback RF (Radio Frequency) signal which is digitally recorded by performing mark width modulation so as to make the linear recording density constant, it is necessary to accurately extract jitter information in order to improve playback performance by optimized playback path.
In recent years, application of a PRML (Partial Response Maximum Likelihood) signal processing technique has been increasing for improvement in playback quality. In this case, a phase of a clock component corresponding to a channel bit frequency possessed by a playback RF signal must be detected from a signal in which an offset component in an amplitude direction is corrected, thereby to realize phase sync pull-in for synchronization of a sampling signal. In this case, it is necessary to extract jitter information from an amplitude component possessed by the sampling signal.
Hereinafter a description will be given of a method for performing detection of a digital binary signal and extraction of jitter information, using a signal synchronized with the phase of the clock component corresponding to the channel bit frequency of the playback RF signal.
With reference to FIG. 15, a playback RF signal 3 that is reproduced from an optical disc medium 1 by a playback signal detection circuit 2 is input to a waveform equalizer 57. The waveform equalizer 57 subjects the input signal to a correction that emphasizes a high-frequency band while emphasizing an output signal, and removes noise components that exist in frequency bands other than a demodulation signal, thereby improving jitters included in the playback RF signal. An analog-to-digital converter (hereinafter referred to as an A/D converter) 5 for converting an analog signal into a digital signal samples the output signal of the waveform equalizer 57 to obtain a multiple-bit digital RF signal 6. The A/D converter 5 employs, as its clock signal, a sampling clock 58 having a channel bit frequency component, which is generated by a voltage-controlled oscillator (hereinafter referred to as a VCO) 63. The channel bit frequency component is a frequency component corresponding to one bit of NRZI (Non Return to Zero Invert) codes which are actually recorded on the optical disc medium, and one channel bit corresponds to “1” or “0” of digital data.
The multiple-bit digital RF signal 6 which is sampled by the A/D converter 5 is input to a band limiting circuit 59, whereby unnecessary low-frequency components included in the digital RF signal 6 are removed. An output signal of the band limiting circuit 59 is output to a phase comparison processing block 60 and to a digital filter 64.
The phase comparison processing block 60 detects a phase error on the basis of the signal inputted to the block 60, and outputs the detected phase error signal to a low-pass filter (hereinafter referred to as a LPF) so as to synchronize the phase components of the digital RF signal 6 and the sampling clock 58. The phase error signal is filtered by the LPF 61, and thereafter, converted into an analog control quantity by a D/A converter 62 for converting a digital signal into an analog signal. The VCO 63 is driven on the basis of the analog control quantity to generate a sampling clock 58. Generation of a sampling clock 58 synchronized with the clock component of the playback RF signal 3 as well as generation of a digital RF signal 6 can be carried out by a feedback loop comprising the A/D converted output of the A/D converter 5→the band limiting circuit 59→a phase comparison processing block 60→the LPF 61→the D/A converter 62 the VCO 63→the sampling clock input of the A/D converter 5.
On the other hand, the digital filter 64 performs waveform equalization so as to output a signal that is obtained by performing a predetermined partial response equalization (PR equalization) for the output signal from the band limiting circuit 59. The equalized output from the digital filter 64 is decoded by a Viterbi decoder 65, thereby generating a maximum likelihood digital binary signal 27.
On the other hand, an absolute value of the phase error and polarity judgement information 67, which are outputted from the phase comparison processing block 60, are input to a jitter detection processing block 68. The jitter detection processing block 68 generates jitter information 69 on the basis of the inputted phase error absolute value 66 and the polarity judgement information 67 (for example, refer to Japanese Published Patent Application No. 2001-250341 (Pages 7˜10, FIGS. 2˜6)) It is possible to improve the playback quality from the optical recording medium by the demodulation of the digital binary signal based on the digital signal processing synchronized with the playback RF signal clock component, and the extraction of the jitter information obtained from the amplitude component.
In the conventional construction mentioned above, however, since a digital division circuit for performing jitter detection operation in the jitter detection processing block 68 is needed to perform high-speed operation, there occurs a limitation on the playback speed at which jitter information can be detected. Further, in order to increase the playback speed, the scale and power consumption of the circuit for extracting jitter information should be increased. On the other hand, there is a playback signal processing method which performs sampling with a cycle twice as long as the channel bit rate, as a method that is effective to reductions in the circuit scale and the power consumption during high-speed playback. However, the conventional construction cannot be applied as it is, to this method.
Furthermore, there may be cases where jitter information cannot be extracted during high-speed playback. Even if jitter information is extracted, there is a risk of degradation in the accuracy of the jitter information. Accordingly, adjustment for performing optimization based on the jitter information becomes insufficient, leading to a high possibility of degradation in playback performance.